Servovalve with accumulator means on drain cavities

ABSTRACT

A servovalve assembly used with hydraulic shakers comprising accumulator means in close association with all low pressure drain and/or return areas of the servovalve to prevent cavitation in the drain cavities and return lines. More accurate dynamic response from the valve to an input program is thus obtained. In the drain or return lines from the main controls spool, accumulators are mounted directly in the valve block to eliminate pressure spikes or peaks caused by cavitation in these normally low pressure areas. The opposite ends of the main spool are also equipped with resilient diaphragm type accumulator means to prevent pressure peaks in these areas. On the pilot spool, the drain area cavities are provided with small O-ring accumulators wherein the O-ring groove is provided with relief areas so that when the spool movement causes pressure fluctuations, the O-rings will deflect into the provided relief areas to prevent high peak pressures. The valve then is capable of operating without disruptive pressure peaks during dynamic operation.

United States Patent Petersen [451 Aug. 1, 1972 [54] SERVOVALVE WITHACCUMULATOR Primary Examiner-William R. Cline MEANS ON DRAIN CAVITIESAttorneyDugger, Peterson, Johnson & Westman [72] Inventor: Niel R.Petersen, Hopkins, Minn. [57] ABSTRACT [73] A tg fi fiz' Corporation Aservovalve assembly used with hydraulic shakers p comprising accumulatormeans in close association [22] Filed: June 12, 1970 with all lowpressure drain and/or return areas of the servovalve to preventcavitation in the drain cavities [211 App! 45654 and return lines. Moreaccurate dynamic response from the valve to an input program is thusobtained. 'i E2 In the drain or return lines from the main controls [58]Fieid 69 625 61 spool, accumulators are mounted directly in the valve137 625 25'1/324 5 block to eliminate pressure spikes or peaks caused by138/DIG 6 cavitation in these normally low pressure areas. The Iopposite ends of the main spool are also equipped with resilientdiaphragm type accumulator means to [56] References Clted preventpressure peaks in these areas. On the pilot UNITED STATES EN spool, thedrain area cavities are provided with small O-rmg accumulators whereinthe O-nng groove is pro- 2,6l4,793 10/1952 Sto rm ..l37/525 vided withrelief areas so that when the spool mve 3,191,626 6/1965 Lelbfritz..137/625.69 causes pressure fluctuations, the wings will 3,477,464ll/l969 Ryan ..137/525 X V deflect into the provided relief areas toprevent i 2,973,746 3/1961 Jupa ..l37/625.63 peak pressures The valvethen is capable of operating 3,189,050 6/1965 Heckrnann ..l37/625.63without disruptive pressure peaks during dynamic 3,209,782 10/1965Wolpin et a1...,.....137/625.69x operation 3,456,688 7/1969 Clark..137/625.63

16 Claims, 6 Drawing Figures PATENTEDAUB H912 SHEET 1 [IF 4 FIE: .'Z

SPLACEM TRANSDUCER CONTROLS INVENTOR. N IEL R. PETERSEN RESERVOIR yATTORNEYS minnows 1 I912 3.680.594

- sum 2 OF 4 ATTORNEYS PATENTEDAus {I912 3.680.594

. sum 3 on} INVENTOR.

NIEL R. PETERSEN BY I ATTORNEYS PKTENTEDAU; I 1912 SHEET Q [If 4 IATTORNEYS SERVOVALVE WITH ACCUMULATOR MEANS ON DRAIN CAVITIES BACKGROUNDOF THE INVENTION has been done as shown in the article entitled Hydrau-5 lie Vibrators by John A. Dickie appearing in Product Engineering,Design Edition, December 9, 1957, pages 94-98. Shake tests usuallyinvolve commanding the servohydraulic actuators with a sine wave signalof the desired frequency. However in the past, servohydraulic shakershave been plagued by very high distortion (frequently over 50 percenttotal harmonic distortion). The result is that whereas the desired testmay be programmed at a certain frequency and amplitude of acceleration,other excitation factors are present which may do substantial specimendamage or otherwise negatively influence the result of the test due todistortion caused by the behavior of the hydraulic fluid under theoscillating flows.

In addition, hydraulic shakers have in the past tended to exhibit aresponse jump phenomena in which the amplitude of the output does notincrease smoothly with the amplitude of the command signal. The resultis that there will exist frequency-amplitude combinations at which theshaker will not be stable. If some sort of automatic excitation levelcontrol is used with the system, operation of the shaker may result in avarying load even if the program is for a constant force level.

Distortion has been found to be combinations of several basic problems.These include:

Spikes on the load waveform caused by collapse of fluid cavitationbubbles in the return lines connecting the servovalve return to the pumpreservoir. The acceleration of the fluid flowing in the return linecauses this cavitation after the valve return ports are opened forreturn flow and then are momentarily closed for reversing the directionof operation of the valve;

Subharmonic oscillations of the output also are caused by cavitationwithin the drain cavities of the servovalve.

This drain cavity cavitation is caused by the motion of the internalpilot and main stage valve spools of the assembly.

The main stage spool requires drain flow at its stub ends to be portedback and forth rapidly as the spool oscillates during operation. Toprevent the possibility of separation in the present device a largediameter cross drilled port is used together with elastic membranesacting as accumulators to absorb rapid fluid acceleration pulses.

In the pilot stage, the ends of the pilot stage spool are at the fulldiameter of the spool. A large cross port drilled hole is included andan accumulator effect is accomplished by distorting the sealing O-ringsused on the drain cavities into cut-outs opening to the O-ring.

Standing waves in the pressure line supplying hydraulic fluid to theservovalve result in undesirable load waveform shapes at moderatefrequencies.

These are minimized by the inclusion of a pressure accumulator with avery large throat size on the pressure line. The pressure lineaccumulators are used to provide a nearly constant supply pressure tothe servovalve pressure metering orifices, as distinguished frompressure line accumulators previously used only to suppress upstreamhydraulic line transients.

Thus while the use of accumulators has been well known on hydraulicdevices, the positioning of the accumulators in a proper relationship toeliminate distortion and non-linear operation has not previously beendone.

SUMMARY OF THE INVENTION The present invention relates to a servovalveconstruction wherein distortion and non-linearities are greatly reduced.Small accumulators are placed directly adjacent the servovalve, in allpressure, return and drain areas. In particular, the main return linefor the servovalve leading to a reservoir is equipped with accumulatorspositioned very closely adjacent to the main valve spool return orificeswhich will prevent cavitation of the hydraulic fluid flowing to thereservoir and will eliminate pressure spikes on the valve.

Accumulator means having outer shells fixed directly to the mainservovalve block and opening directly into the return orifices of theservovalve are provided so that the accumulators are as close aspossible to and practically overlie the return orifices of the mainvalve spool. Further, the drain cavities of the main valve spool havediaphragm type accumulators and at the drain cavities of the pilotvalve, where O-rings are used for seal, the O-rings are mounted ingrooves having reliefs or scallops which permit the O-ring to deflectslightly in a direction so that it does not interfere with the scalingfunction of the O-ring, but forms a small accumulator to smooth outoperation. This invention also comprises the forming of smallaccumulators with 0- rings without disturbing the sealingproperties ofthe rings.

It is an object of the present invention to reduce nonlinearities in aservovalve by proper location accumulator means.

It is a further object of the invention to present a new small volumeO-ring accumulator for general use in hydraulic devices.

Other objects will become apparent as the description proceeds.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a part schematicrepresentation of a typical shake testing arrangement showing aservovalve made according to the present invention installed in thesystem;

FIG. 2 is an enlarged sectional view taken as on line 2-2 in FIG. 1 withparts broken away;

FIG. 3 is a sectional view taken as on line 3-3 in FIG. 2;

FIG. 4 is a sectional view taken as on line 4-4 in FIG. 2;

FIG. 5 is an enlarged side elevational view of a portion of the seal fora drain cavity of the pilot spool of the servovalve assembly showing aconstruction to get fluid absorption with this device; and

FIG. 6 is a fragmentary sectional view taken as on line 6-6 in FIG. 2.

A servovalve assembly illustrated generally at 10, as shown, is used incombination with a shake test machine. The set-up as shown schematicallyincludes a seismic base 12 which supports a double acting, hydraulicactuator 17. The actuator has a rod 13 supporting a specimen to betested shown schematically at 14. The actuator assembly 17 has aninternal piston attached to the rod 13. Substantially non-compressiblehydraulic fluid under pressure supplied to opposite ends of thehydraulic actuator 17 acts on the piston and thereby moves the rod. lnshake testing the device is programmed to cyclically move the rod in anoscillating motion shown by double arrow 15. The actuator is cycled atthe desired frequency, usually as shown from 1 to 1,000 cyclespersecond, and at acceleration levels from to 100 G5. Although theapplication shown is a shaker testing system the invention describedherein can be adapted to other specimen testing configurations. Theservovalve assembly is programmed to direct fluid under pressure to thehydraulic actuator at the desiredfrequency. A hydraulic pump 20 forsupplying hydraulic fluid under pressure receives the hydraulic fluidfrom a reservoir 21 and supplies this fluid under pressure through apressure line 22 to a pressure port 23 of a valve block or body 24 ofthe servovalve assembly. The servovalve assembly 10 then is operated todirect fluid under pressure either to a first actuator pressure line 25or a second actuator pressure line 26 leading from controlled ports inthe valve block to the actuator by way of a matching manifold 58, tomove the rod 13 in the desired direction. When one of the actuatorpressure lines is under pressure, the other is simultaneously connectedto return.

A displacement transducer 27 is used in conjunction with the actuator 17and delivers a controllable signal indicating actuator rod displacementalong a line 28 to a control console 31 that is of a desired type toinitiate drive signals for appropriate excitation of the servovalve.

The controls for the servovalve are commonly used and supply asinusoidal signal to cause oscillatory motion. The displacementtransducer provides a feedback signal to ensure that the actuatoroperates at its desired displacement.

In addition to the hydraulic lines previously mentioned, a hydraulicpressure line 33 comes from the pump to the valve block 24 and leadsthrough passageways as shown in FIG. 2 to a pilot spool valve assemblygenerally shown at 35. The pilot valve assembly is operated in responseto electrical signals coming from the control console 31 to open orclose the pilot stage pressure ports to direct fluid under pressurethrough provided passageways (shown in dotted lines) in the usual mannerup to the slave or main spool assembly shown generally at 40. The slavespool assembly, including a spool 49 which is the main spool of thehydraulic servovalve, and the actuating chambers 41 .and 42 for the mainspool 49, is perhaps best shown v 'its sleeve 46. Return or centeringsprings 45 are positioned at opposite ends of the pilot spool. Note thata conical member is integral with the pilot spool and has an outerflange portion that carries the moving coil. The wires which carrycurrent to the moving coil are shown attached to the conical portion.The pilot spool valve is a four-way valve, and when, for example, the

spool 44 is shifted in one direction longitudinally, for exampleupwardly when viewed in FIG. 2, (away from the field coil end) hydraulicfluid under pressure from the pressure line 33 will flow through theprovided ports in the sleeve 46 for the pilot spool 44 through apassageway 47 indicated in dotted lines in FIG. 6 to the actuatingchamber .41 at one end of the main spool 49. This chamber 41 is betweenan outer sleeve 48 and the stub end of the main spool 49. Simultaneouslythe chamber 42 'will be ported to return through a passageway and portsof the pilot valve and to the reservoir by way of the pilot return line34.

When fluid under pressure is introduced to the chamber 41, the mainspool 49 is moved in axial direction indicated by the arrow 52. Fluidunder pressure is then directed from the internal pressure port 53 shownin FIG. 3 into actuator line passageway 54 which is connected to line26, for example. The spool movement will cause fluid under pressure toflow into the actuator 17 by way of the manifold 58 from the pump andmounted directly onto the valve block 24 with a suitableclamping'assembly illustrated generally at 61 including bolts 62. Theaccumulator has an interior bladder 65 and has a wide-mouth in which aperforated bladder restrainer 63 is sealingly positioned. The restrainer63 permits free passage of hydraulic fluid, but holds the bladder in theouter bottle.

The accumulator 60 is actually open directly to the return passageway 56inside the valve block. Thus,

when the return oil flows through the line 25 of the actuator, in thepassageway 55, and through the, main spool assembly and out thepassageway 56, the accumulator, which is recharged at a relatively lowair pressure setting will provide a compressible fluid cushion over thesubstantially non-compressible hydraulic fluid. When the spool 49 closesoff flow through passage 56 suddenly as the valve cycles, theaccumulator prevents cavitation in the return line caused by stoppingflow of the hydraulic oil in direction toward the reservoir.

Compressed fluid in the accumulator expands to.

prevent separation and cavitation of the hydraulic fluid in the returnlines. The accumulator mouth is open directly to the passageway so theaccumulator acts to prevent any cavitation even right next to the mainvalve.

To move the main valve spool 49 in the direction opposite from the arrow52, a signal comes from the controls 31 through the line 32, whichdrives the voice coil 39 to move the pilot spool 44 in direction so thatfluid will flow from the pressure line 33 through the passageways andpast the lands provided on the pilot valve spool, through a passageway69 shown in dotted lines, into the main spool actuating chamber 42. Atthe same time, the pilot valve opens passageways to drain chamber 41will pass back through the pilot drain line 34 to the reservoir. Thusthe pressure in chamber 42 will move the main spool in oppositedirection from the arrow 52. The pilot spool controls the main spool sothat the small movements of the pilot spool control the fluid underpressure going to the actuator.

When the main valve spool changes position because of a reverseactuation signal to the pilot valve, the main spool 49 will move veryrapidly in opposite direction from arrow 52 and will then very quicklyshut off the flow through passageway 56 to the reservoir. The velocityof the hydraulic fluid flowing through the lines 57 tends to keep thefluid moving for a short while even after the valve is closed. Thiscauses separation in a closed system. The accumulator 60 will preventthis cavitation and will thus prevent high force peaks which occur whenthe cavitation bubbles collapse as the velocity of the oil flowing backto the reservoir drops. The pressure spikes in the return line reflectback to the actuator cavities causing spikes on the actuator wave form.

In cyclic operation, after the main valve spool 49 moves in oppositedirection from that indicated by the arrow 52, hydraulic fluid underpressure from the pump ports 53 will be admitted past a land on thespool to the passageway 55, and thus into the line 25 to move theactuator 17 in opposite direction from the previous direction. Thepassageway 54 will be opened by a land on spool 49 to the returnpassageway 66 which opens to the return line 67. Fluid then will flowfrom the base end of actuator 17, through line 26 and out through thepassageway 66 into the return line 67 back to the reservoir. As shown,passageway 66 is open through a wide port to a second accumulator 72held in place with the same framework as the accumulator 60. Theaccumulator 72 includes an interior bladder 73. This accumulator is opento the passageway 66 on the return side of the main spool 49 to preventcavitation and surges of pressure in the return lines 67 and 68 causedby the velocity of the oil in the return lines during very rapid mainvalve spool actuation. The accumulator 72 is operating at a lowprecharge pressure, about 15 psi gage, to keep a suitable compressiblefluid pressure against the bladder 73. The accumulator will preventpressure peaks due to collapsing bubbles caused from cavitation when themain spool 49 is closed and again operated in direction as indicated bythe arrow 52. The main spool actuation is at a desired frequency whichcan be quite high, for example in the range of 250 cycles per second,and this can give very high pressure and force peaks and undesirablepressure wave forms which might otherwise reflect on the motion of theactuator piston rod 17 causing distortion. A perforated bladderrestrainer screen 74 is also mounted at the mouth of the accumulator 72.

A feedback signal is provided from the main spool with a suitable signalmeans illustrated generally at 75, and a connection leading to the line76 which does back into the suitable controls 31 for determining thedynamic position of the main spool, this signal being used in thedetailed function of the controls 31. Maximum spool travel is plus orminus a small fraction of an inch of the main spool 49 so that movementsare very short, but high and rapidly changing flows are involved. Thismeans that the length of return lines have a substantial amount ofhydraulic fluid and above only a few cycles per second the inertia ofthe fluid in the return lines becomes important.

The actuation of the pilot spool 44 and the main spool 49 isconventional, but the addition of the accumulators 60 and 72 onto thereturn passageways 56 and 66 prevents cavitation even at highfrequencies of operation.

In order to further smooth out the operation of the valve assemblies,the drain areas at the ends of both the main spool and the pilot spoolare provided with means for absorbing pressure peaks and thus greatlyreducing the amplitudes of extraneous pressure spikes and smoothing outthe cyclically actuated operation.

First, in the pilot spool area, it can be seen that the pilot spool 44is integral with the frustoconical voice coil carrying member 75.

The voice coil carrying member 75 is in a chamber 76 defined in thevalve block, which is at drain or return pressure, and suitable portingis provided for connecting this chamber in the valve block leading fromthis chamber 76 to the reservoir.

At the opposite end of the pilot spool, there is a sleeve 77 mounted inan opening in the valve block which houses the zero adjust screw 78 forthe pilot stage valve. The sleeve is held in the valve block with asuitable member, and an O-ring 79 seals the sleeve with respect to thechamber that it is mounted in.

The sleeve 77 is made specially to form pulsation dampening in thispilot spool drain area chamber 81. The chamber 81 is formed between theinner end portions of the sleeve 77 and the interior surfaces of thepilot valve and pilot valve sleeve. The balancing spring 45 at this endof the pilot valve is mounted inside an interior opening of the sleeve77. As shown perhaps best in FIG. 5, the end of the sleeve 77 isprovided with short slots 82. The O-ring 79 is mounted in an annulargroove 83 on the end portion of the sleeve. The O-ring here is sealingon the interior surface of the opening for the sleeve 77 and the innerbottom surface of groove 83. As shown, in order to provide for somepulsation dampening, the groove 83 has recesses 84 extending indirection along the longitudinal axis of the sleeve 77, defined thereinon an opposite side of the O-ring 79 from the chamber 81. In ordinaryoperation the O-ring 79, which is under some tension, will lie along aradial plane. When pressure against the O-ring 79 from chamber 81increases, the pressure will force the O- ring back toward the exteriorend of the screw 78, and the sides of the O-ring 79 will deflect fromthe normal planar position slightly into these scallops or recesses 84,as shown in FIG. 5. The seal surfaces of the O-ring remain in contactwith the inner surface of the chamber for sleeve 77 and groove 83 so thechamber 81 doesnt leak. Because of the low quantities of displaced oilin the chambers this is a sufficient amount of an accumulator action toprevent pressure spikes and cavitation from occurring within thischamber. When the pressure in the chamber 81 reduces, the elasticity ofthe O-ring returns to its normal position. Thus, the O-ring 79 itselfserves as a peak pressure relief means at the drain area of this end ofthe pilot spool and serves as a seal as well. The recesses 84 are atatmospheric pressure and can be vented if desired.

At the voice coil end of the pilot spool, as shown, the chamber 76 issealed by the housing 86 for the field coil that bolts against thesurface of the block 24 for the servovalve. An O-ring 88 is provided andis placed in an annular groove that extends annularly around the outerperiphery of the chamber 76. In this particular instance, referring toFIG. 6, the O-ring recess is provided with radially extending recesses87. The O-ring 88 can thus deflect elastically out of its normal annularshape into these recesses 87. The O-ring 88 here seals on its lateralsides and the sealing surfaces remain in contact with the O-ring 88 asit deflects into the recesses 87. At both ends of the pilot spool theO-ring grooves are provided with recesses that open to the respectivegrooves. Note that the recesses 84 and 87 are partially defined bysurfaces that form a continuation of one sealing surfaces for therespective O-ring. This permits O-ring deflection without losing thesealing properties. Because the chamber 76 is ported to the reservoir,and because the dynamic volume involved is quite low,the smalldeflections of the O-ring 88 into the recesses 87 provide a suflicientaccumulator action. The scallops or recesses'84 and 87 are vented toatmospheric pressure and do not have to be sealed.

The drain chambers for the pilot spool are connected together with apassageway 89, shown in FIG. 6. The drain chambers also are connected tothe reservoir 21 by suitable passageways.

' The required drain areas at opposite ends of the main spool extendoutwardly beyond the stub ends of the main spool and are shown aschambers 90, and 91, respectively. These chambers are connected with aninternal passageway 101. The passageway 101 leads into the reservoirthrough a suitable internal passageway and the return lines leading fromthe valve block. The operation of the main spool 49 requires a rapidoscillation of oil in these drain areas. When the main spool 49 isoperating with an oscillating motion along its longitudinal axis, theoil in these drain chambers will be pulsating back and forth along withthe movement of the spool. The drain chamber 90 at one end of the mainspool is defined in part by a resilient elastomeric member 92 forming asurface held in place with a clamp 93 that is bolted onto a main portionof the feedback mechanism. The chamber 90 is thus able to change involume by deflecting the member 92 with respect to a chamber 94 definedin the cap 93. This chamber 94 is filled with a compressible fluid suchas air that is vented to atmosphere, and so the membrane 92 provides asmall accumulator effect in this drain area.

At the opposite end of the main spool, chamber 91 is defined in part bya resilient elastomeric member 95 held in place with a cap 96. Thechamber 91 is thus in part defined by the member 95 that will deflect.The cap 96 has an interior chamber 97 defined therein that is vented toatmosphere, and the member 95 then can deflect into the chamber 97 togive an accumulator effeet to prevent cavitation and pressure spikesfrom occurring in the drain area at this end of the main spool. Evenrapid oscillations, where the inertia and accelerations of the fluids inthe drain area chambers 90 and 91 becomes high, the resilientelastomeric members 92 and will deflect into their respectiveatmospheric chamber areas to cushion rapid changes in pressure andprevent cavitation by deflecting suitably to accommodate fluctuations inpressure.

In addition, a suitable accumulator 100, shown schematically, is placedonto the pressure line 25 leading to I the servovalve 10 in order tohave a full cushioned servovalve operation. The accumulator 100 shownschematically on the pressure line, is a conventional type accumulatorincluding an outer wall having an inner fluid barrier that is subjectedto pressure, and open to the hydraulic line. This accumulator, however,has a .unit thus becomes a fully conditioned servovalve thus accumulatormeans at the main drain lines, on the pressure lines, and at thenon-working drain cavities where fluid is oscillated on the interior ofthe unit.

in previous devices where accumulators were attached to the return linesbut at some distance from the block, there was sufficient hydraulicfluidvolume in the return lines between the servovalve and the accumulatorsso that cavitation could occur between the internal passages of theservovalve spool and the accumulator. The present structure preventsthis because the accumulators are open right directly to the spoolpassageways from the servovalve and prevent separation and cavitationand resulting peak pressures attendant thereto. in 4 addition, where amulti-stage servovalve is utilized such as in the present invention, the

low pressure drain areas at opposite ends of both the pilot valve spooland the main valve spool are provided with small deflectable means toprevent cavitation by absorbing, fluid pulses. in the small flow areas,distorting of the sealing O-ring grooves either into radially extendingrecesses or in axially extending recesses equally as small which do notdisturb the sealing properties of the O-ring but permit some deflectioninto these cavities under higher pressures, provide the necessarydamping effect on fluid pulses. Note that the O-rings have some surfaceareas open to the chambers they seal. Thus, the chambers 81 and 76 arepartially defined by a deflectable elastomeric member.

The smoother servovalve response occasioned by eliminating the abruptnon-linearities caused by internal cavitation when the fluid massfollowing the spools tends to separate, provides for the valve outputamplitude to be modulated smoothly and this allows for closer electroniccontrol of the level of output put on by the actuator 17 at highfrequencies. 5

It should be noted that the controls 31 are nowcommercially availablefor use in conjunction with servovalves controlled shake test machinesfrom MTS Systems Corporation, Eden Prairie, Minnesota, and include meansfor generating the necessary control signal for the servovalve from thedesired program, and feed back means for maintaining the displacementand frequency at the programmed level. The system uses a hydraulic oilfor the actuating fluid.

The small O-ring accumulators can be used in many types of hydraulicdevices, as long as the O-ring will deflect elastically into an openarea under pressure pulsation and then elastically return to its normalposition when the pressure reduces. The O-ring accumulator finds specialusage in the drain areas of servovalves where cyclic loading is present.

What is claimed is:

1. In a hydraulic device having an oscillating control element, draincavity means formed in said hydraulic device and open to at least oneportion of said oscillating element and to a drain, said drain cavitymeans including a groove defined in said device and having two facingsealing surfaces, an elastomeric ring member positioned in said groovemeans and engaging the sealing surfaces, a wall portion of saidelastomeric ring defining a wall portion of said drain cavity means, andrelief cavity means defined in said hydraulic device and open to saidgroove at spaced locations on an opposite side of said ring member thanthe wall portion of said ring defining a wall portion of said draincavity means, said relief cavity means being of size to permit said ringmember to deflect under increased elastomeric tension due to pressure insaid drain cavity means while remaining in contact with the sealingsurfaces to keep said drain cavity sealed.

2. The combination as specified in claim 1 wherein said groove means forsaid ring member comprises an annular groove generated about an axis,and said relief cavity means extend radially outwardly from said groovewith respect to said axis at spaced intervals around said annulargroove.

3. The combination as specified in claim 1 wherein said groove means forsaid ring member comprises an annular groove generated about an axis,and said relief cavity means is open to said groove at spaced intervalsaround said ring and extend in generally parallel to said axisdirection.

4. The combination as specified in claim 1 wherein said oscillatingcontrol element comprises a valve spool, and said drain cavity means isopen to at least one end of said spool.

5. The combination as specified in claim 4 wherein said drain cavitymeans is open to opposite ends of said valve spool and passage means topermit movement of hydraulic fluid between said opposite ends as saidspool oscillates.

6. In a hydraulic device comprising a valve member having valve spoolcontrol means movable in opposite directions to control flow ofsubstantially noncompressible hydraulic fluid, chamber means on at leastone end of said spool, said chamber means being maintained at drainpressure and providing drain means for said spool, said chamber meansbeing defined at least in part by an elastomeric member forming at leasta portion of the wall of said chamber means and being movable from arest position only under pressure differentials thereon which overcomeelastomeric resistance of said member, and at least a portion of anopposite side of said elastomeric member from said chamber means beingopen to a compressible fluid to permit said elastomeric member todeflect and compress said compressible fluid to absorb transientpressure changes in the noncompressible hydraulic fluid in said chamberengaging said elastomeric member.

7. The combination as specified in claim 6 wherein said elastomericmember is a disc-like member and forms a substantial portion at one wallof said chamber.

8. The combination as specified in claim 6 wherein said elastomericmember is an annular ring member, and is open to said chamber along oneportion of said ring type member. 1

9. The combination as specified in claim 8 wherein the portions of saidelastomeric ring member open to a compressible fluid pressure compriserelief chambers spaced around the annular periphery of said ring memberon a side opposite of said ring member from said chamber.

10. The combination as specified in claim 6 wherein said valve member isan electro-hydraulic servovalve having a pilot valve spool control and amain valve spool, and wherein said servovalve has main valve drainchambers defined at opposite ends of said main valve spool, passagewaymeans connecting said main valve drain chambers, and said pilot valvehas pilot valve drain chambers at opposite ends of said pilot valvespool, passageway means connecting said pilot valve drain chambers, andall of said drain chambers comprising said chamber means.

11. A servovalve having a valve block mounting a pilot stage valve and amain stage valve, said main stage valve being operated in oppositedirections in response to said pilot stage valve to control flow ofsubstantially noncompressible hydraulic fluid to and from an actuator,said valve block further including internal drain passage meanscontrolled by said main stage valve to permit drain fluid flow from saidactuator through said passage means to the reservoir as said actuator isoperated and accumulator means in said drain passage means including abarrier member open to the drain passage means on a surface thereof, andcompressible fluid on an opposite surface of said barrier from the drainpassage means to permit said barrier to move under pressure transientsin said noncompressible fluid, said accumulator means opening to thedrain passage means closely adjacent said internal drain passage meansof said servovalve.

12. The combination as specified in claim 11 wherein said main stage ofsaid servovalve is defined in a valve block, and wherein saidaccumulator means for each of said internal drain passage means compriseseparate accumulator members having outer housings, and havingaccumulator inlets, said accumulator inlets being mounted to openthrough said valve block to said drain passage means.

13. The combination as specified in claim 12 wherein remote drain linesfor hydraulic fluid are connected to said drain passage means at portson said valve block, and wherein said accumulator means open throughdifferent areas of said valve blocks to said drain passage means thansaid ports.

14. In a servovalve assembly for controlling substantiallynoncompressible hydraulic fluid under pressure and having a linearlymovable pilot valve means, a linearly movable main stage valve means,and return line means leading to reservoir from said main stage valvemeans, and having hydraulic fluid drain cavities at opposite ends ofsaid linearly movable main stage valve means and said linearly movablepilot valve means, the improvement comprising a pneumatic accumulatormeans open to said return line means closely adjacent said main stagevalve, and separate accumulator means open to each of the drain cavitiesin said ser vovalve to absorb transient pressure peaks caused by inertiaof the hydraulic fluid in said drain cavities when the respective valvemeans move linearly.

15. The combination as specified in claim 14 wherein said pilot stagecomprises a pilot stage valve spool, and wherein there are draincavities at opposite ends of said pilot stage valve spool, and whereinsaid accumulator means on the drain cavities at opposite ends of saidpilot stage valve spool comprises elastic ring means normally sealingsaid pilot stage cavities, and relief cavities adjacent and open to saidelastic ring means and into which said elastic ring may elasticallydeflect when pressure in said pilot stage drain cavities exceeds apredetermined amount.

16. The combination as specified in claim 14 and a I source of hydraulicfluid under pressure, and wherein said servovalve has pressure outletport pressure lines leading from the source of fluid under pressure tosaid servovalve and wide-mouth accumulator means being sufficient toprovide substantially constant supply pressure to the servovalvepressure inlet ports as the servovalve is operated.

1. In a hydraulic device having an oscillating control element, draincavity means formed in said hydraulic device and open to at least oneportion of said oscillating element and to a drain, said drain cavitymeans including a groove defined in said device and having two facingsealing surfaces, an elastomeric ring member positioned in said groovemeans and engaging the sealing surfaces, a wall portion of saidelastomeric ring defining a wall portion of said drain cavity means, andrelief cavity means defined in said hydraulic device and open to saidgroove at spaced locations on an opposite side of said ring member thanthe wall portion of said ring defining a wall portion of said draincavity means, said relief cavity means being of size to permit said ringmember to deflect under increased elastomeric tension due to pressure insaid drain cavity means while remaining in contact with the sealingsurfaces to keep said drain cavity sealed.
 2. The combination asspecified in claim 1 wherein said groove means for said ring membercomprises an annular groove generated about an axis, and said reliefcavity means extend radially outwardly from said groove with respect tosaid axis at spaced intervals around said annular groove.
 3. Thecombination as specified in claim 1 wherein said groove means for saidring member comprises an annular groove generated about an axis, andsaid relief cavity means is open to said groove at spaced intervalsaround said ring and extend in generally parallel to said axisdirection.
 4. The combination as specified in claim 1 wherein saidoscillating control element comprises a valve spool, and said draincavity means is open to at least one end of said spool.
 5. Thecombination as specified in claim 4 wherein said drain cavity means isopen to opposite ends of said valve spool and passage means to permitmovement of hydraulic fluid between said opposite ends as said spooloscillates.
 6. In a hydraulic device comprising a valve member havingvalve spool control means movable in opposite directions to control flowof substantially noncompressible hydraulic fluid, chamber means on atleast one end of said spool, said chamber means being maintained atdrain pressure and providing drain means for said spool, said chambermeans being defined at least in part by an elastomeric member forming atleast a portion of the wall of said chamber means and being movable froma rest position only under pressure differentials thereon which overcomeelastomeric resistance of said member, and at least a portion of anopposite side of said elastomeric member from said chamber means beingopen to a compressible fluid to permit said elastomeric member todeflect and compress said compressible fluid to absorb transientpressure changes in the noncompressible hydraulic fluid in said chamberengaging said elastomeric member.
 7. The combination as specified inclaim 6 wherein said elastomeric member is a disc-like member and formsa substantial portiOn at one wall of said chamber.
 8. The combination asspecified in claim 6 wherein said elastomeric member is an annular ringmember, and is open to said chamber along one portion of said ring typemember.
 9. The combination as specified in claim 8 wherein the portionsof said elastomeric ring member open to a compressible fluid pressurecomprise relief chambers spaced around the annular periphery of saidring member on a side opposite of said ring member from said chamber.10. The combination as specified in claim 6 wherein said valve member isan electro-hydraulic servovalve having a pilot valve spool control and amain valve spool, and wherein said servovalve has main valve drainchambers defined at opposite ends of said main valve spool, passagewaymeans connecting said main valve drain chambers, and said pilot valvehas pilot valve drain chambers at opposite ends of said pilot valvespool, passageway means connecting said pilot valve drain chambers, andall of said drain chambers comprising said chamber means.
 11. Aservovalve having a valve block mounting a pilot stage valve and a mainstage valve, said main stage valve being operated in opposite directionsin response to said pilot stage valve to control flow of substantiallynoncompressible hydraulic fluid to and from an actuator, said valveblock further including internal drain passage means controlled by saidmain stage valve to permit drain fluid flow from said actuator throughsaid passage means to the reservoir as said actuator is operated andaccumulator means in said drain passage means including a barrier memberopen to the drain passage means on a surface thereof, and compressiblefluid on an opposite surface of said barrier from the drain passagemeans to permit said barrier to move under pressure transients in saidnoncompressible fluid, said accumulator means opening to the drainpassage means closely adjacent said internal drain passage means of saidservovalve.
 12. The combination as specified in claim 11 wherein saidmain stage of said servovalve is defined in a valve block, and whereinsaid accumulator means for each of said internal drain passage meanscomprise separate accumulator members having outer housings, and havingaccumulator inlets, said accumulator inlets being mounted to openthrough said valve block to said drain passage means.
 13. Thecombination as specified in claim 12 wherein remote drain lines forhydraulic fluid are connected to said drain passage means at ports onsaid valve block, and wherein said accumulator means open throughdifferent areas of said valve blocks to said drain passage means thansaid ports.
 14. In a servovalve assembly for controlling substantiallynoncompressible hydraulic fluid under pressure and having a linearlymovable pilot valve means, a linearly movable main stage valve means,and return line means leading to reservoir from said main stage valvemeans, and having hydraulic fluid drain cavities at opposite ends ofsaid linearly movable main stage valve means and said linearly movablepilot valve means, the improvement comprising a pneumatic accumulatormeans open to said return line means closely adjacent said main stagevalve, and separate accumulator means open to each of the drain cavitiesin said servovalve to absorb transient pressure peaks caused by inertiaof the hydraulic fluid in said drain cavities when the respective valvemeans move linearly.
 15. The combination as specified in claim 14wherein said pilot stage comprises a pilot stage valve spool, andwherein there are drain cavities at opposite ends of said pilot stagevalve spool, and wherein said accumulator means on the drain cavities atopposite ends of said pilot stage valve spool comprises elastic ringmeans normally sealing said pilot stage cavities, and relief cavitiesadjacent and open to said elastic ring means and into which said elasticring may elastically deflect when pressure in said pilot stage draincavities exceeds a predetermined amount.
 16. The combinaTion asspecified in claim 14 and a source of hydraulic fluid under pressure,and wherein said servovalve has pressure outlet port pressure linesleading from the source of fluid under pressure to said servovalve andwide-mouth accumulator means being sufficient to provide substantiallyconstant supply pressure to the servovalve pressure inlet ports as theservovalve is operated.